Commercial rubber goods generally include nitrogen-containing materials which are used as, or formed from, curing or accelerating agents.
Dithiocarbamates, e.g. of the formula (R.sub.a R.sub.b N--CSS).sub.y M, and thiuram sulphides, e.g. of the formula R.sub.a R.sub.b N--CS--S.sub.z --CS--NR.sub.a R.sub.b, in which y and z are each integers, R.sub.a and R.sub.b are each aryl, alkyl or substituted alkyl and M is a metal such as sodium or zinc, are widely used as accelerators and curing agents for rubber goods. Nitrosamines of the formula R.sub.a R.sub.b NNO are known to be present in, and formed from, corresponding dithiocarbamates and thiuram sulphides. Nitrosamines can also be formed from other N-containing compounds used in rubber processing, e.g. sulphenamides and thioureas.
EPDM (ethylene-propylene-diene-modified) rubber is conventionally cured using a combination of dipentamethylenethiuram hexasulphide, tetramethylthiuram disulphide and tellurium diethyldithiocarbamate. Neoprene is conventionally cured using zinc oxide activated with a thiourea (e.g. ethylenethiourea or diethylthiourea) or a tertiary amine (e.g. hexamethylenetetramine). The given N-containing activators/curing agents are all nitrosatables.
The nitrosamines are recognised to be carcinogenic in animals. Among those nitrosamines tested, only those with exceptional structures, e.g. where R.sub.a or R.sub.b is a tertiary group, have been found to lack carcinogenic activity; see Drucker et al, Zeitschrift fuer Krebsforschung 69 (1967) 103-201 and Lijins et al, J. Nat. Cancer Inst. 49 (1972) 1329-1249. Nitrosamines in which R.sub.a and R.sub.b are each methyl, ethyl, propyl or butyl groups, or R.sub.a and R.sub.b together represent the pentamethylene group, are particularly potent.
Nitrosamines are of potential danger wherever they can come into contact with foodstuffs or sensitive skin, or wherever they can build up in a closed atmosphere, e.g. inside submarines, diving gear or parked cars. There is a risk of exposure to nitrosamines during the manufacture of dithiocarbamates and related compounds, and during their use in manufacturing rubber goods. The presence of nitrosamines in rubber goods is potentially of risk to humans.
Rubber of the type used for baby teats and soothers typically has a nitrosamine content of about 0.5 ppm. Very careful formulation and washing is needed to reduce this concentration to an acceptable level for babies, i.e. below 0.01 ppm. However, because dithiocarbamate accelerators and related compounds are usually used in amounts of about 1% by weight of rubber, there remains, after curing and washing, a substantial residue of N-containing material capable of nitrosation by nitrite in saliva.
Other undesirable N-containing materials are used in rubber processing, including thioureas. Such toxic compounds, and also isocyanates and isothiocyanates, are formed by thermal breakdown in common N-containing curing systems, and are present in the fumes from rubber processing operations.